Determination of the position of the rotor of an electric machine

ABSTRACT

A frequency converter and a method for determining the position of the rotor of an electric machine are provided. The frequency converter includes a load bridge and a control of the load bridge, for supplying electricity between the load bridge and an electric machine connected to the load bridge. The frequency converter includes a determination for at least one electrical parameter of the electric machine, and includes a determination for the position of the rotor of the electric machine. The load bridge is fitted to supply a first alternating electricity excitation signal, which is formed in relation to the electrical angle of the electric machine, to the electric machine. The frequency converter is further fitted to determine the first alternating electricity response signal corresponding to the first alternating electricity excitation signal, and the position of the rotor is determined on the basis of the first alternating electricity response signal.

This application is a Continuation application of co-pending applicationSer. No. 12/897,506 filed on Oct. 4, 2010, which is a continuation of anational phase of PCT International Application No. PCT/FI2009/000043filed on Apr. 1, 2009, and which claims priority to Application No.FI20080318 filed in Finland on Apr. 24, 2008. The entire contents of allof the above applications are hereby incorporated by reference.

The object of the invention is to provide a frequency converter, anelectric drive, and a method for determining the position of the rotorof an electric machine.

In the regulation of an electric machine, the position of the rotor isconventionally identified with an absolute encoder, such as a resolver.Recently, different sensorless identifications of the position have alsobeen developed, which are based on e.g. measurement of the inductance ofthe magnetic circuit of the electric machine as well as on estimation ofthe source voltage of the motor.

Of the control methods of an electric machine, e.g. vector regulationmethods generally require identification of the starting position of therotor, especially in synchronous machine drives. A position erroroccurring at the start of the run and at low speeds might result inuncontrolled behavior of the motor and thus to a hazardous situation.

Methods are known to prior art in which the starting position betweenthe rotor and the stator is synchronized by supplying direct current tothe stator winding and by releasing the magnetized rotor to freely move,in which case the rotor endeavors to turn according to the statormagnetization. In this case a problem is the initial swing of the rotor,which depending on the application may cause deterioration of drivecomfort or can even be actually dangerous.

Methods have also been developed in which the starting position of therotor is determined by measuring the variation of the inductance of themagnetic circuit of the electric machine. This type of method ispresented e.g. in the publication “Peter B. Schmidt, Michael L. Gasperi,Glen Ray, Ajith H. Wijenayake: Initial Rotor Angle Detection Of ANon-Salient Pole Permanent Magnet Synchronous Machine” IEEE IndustryApplication Society, Annual Meeting, New Orleans, La., Oct. 5-9, 1997.The publication referred to presents an identification of the positionof the rotor of a permanent-magnet motor, wherein a pulse-like voltagesignal with the determined values of the electrical angle of the motoris supplied to the stator winding of the permanent-magnet motor as anexcitation, and the current response signals produced by the suppliedpulse-like voltage signals are measured. The inductance of the electricmachine can in this case be determined from the current responses. Whenthe measurement is repeated with sufficiently many different values ofelectrical angle, the variation of the inductance can be determined.Since the variation of the inductance is based on, among other things,saturation phenomena of the magnetic circuit cause by the rotormagnetization, thus also the position between the rotor and the statorcan be determined.

A problem with the aforementioned determination of the rotor position isthat the pulse-like voltage signals and their current response signalsproduce a loud disturbing noise in the electric machine. The use of anelectric machine controlled in this way e.g. in residential buildingscan, in fact, be disturbing, and may require sound insulation of theelectric machine. Problems may occur e.g. in elevator systems in whichthe hoisting machine is controlled with the aforementioned method. Moreparticularly a problem may occur in elevator systems without machinerooms, in which the hoisting machine is disposed in the elevatorhoistway of the building. In addition, a problem in the aforementioneddetermination of the position of the rotor is the measurement of theinductance must be performed by supplying a pulse-like voltage signalseparately with many different values of the electrical angle to achievesufficient accuracy, which lengthens the measuring and at the same timethe duration of the noise caused by the measuring is lengthened.

Publication U.S. Pat. No. 6,401,875 B1 presents an identification of theposition of the rotor of a permanent-magnet motor, wherein a currentsignal is supplied to the stator winding of the permanent-magnet motorseparately with a number of different values of the electrical angle,and the supply voltage signals corresponding to the supplied currentsignals are measured. In this case the inductance of the electricmachine can be determined from the supply voltage signals. When themeasurement is repeated with sufficiently many different values ofelectrical angle, the variation of the inductance can be determined.Since the variation of the inductance is based on, among other things,saturation phenomena of the magnetic circuit cause by the rotormagnetization, thus also the position between the rotor and the statorcan be determined.

The purpose of this invention is to solve the problems presented abovein the description of prior art as well as the problems disclosed in thedescription of the invention below. In this case a determination of theposition of the rotor of an electric machine that is quieter and fasterthan prior art is disclosed. By means of the determination of theposition of the rotor of an electric machine according to the inventionit is possible to determine e.g. the initial angle of the rotor forcontrolling the electric machine.

Some inventive embodiments are also discussed in the descriptive sectionof the present application. The inventive content of the application canalso be defined differently than in the claims presented below. Theinventive content may also consist of several separate inventions,especially if the invention is considered in the light of expressions orimplicit sub-tasks or from the point of view of advantages or categoriesof advantages achieved. In this case, some of the attributes containedin the claims below may be superfluous from the point of view ofseparate inventive concepts.

The frequency converter according to the invention can be e.g. afrequency converter with a current intermediate circuit, a frequencyconverter with a voltage intermediate circuit and a matrix converter.

The electric machine according to the invention can be e.g. an electricmotor or a generator. In this case the electric machine can be e.g. asynchronous machine with a rotor winding or magnetized with permanentmagnets, or a direct-current machine without brushes. The electricmachine can also be e.g. a step motor or a reluctance motor. Theelectric machine can be rotating or it can also be fitted to operate onthe linear motor principle.

In one embodiment of the invention the motor drive is fitted to move thetransport appliance of a transport system. This type of transport systemcan be e.g. an elevator system, an escalator system, a travelatorsystem, a direct drive elevator system, a crane system or a vehiclesystem. If a motor drive is fitted to the elevator system, the electricmachine can also comprise a traction sheave connected to the hoistingrope or hoisting belt of the elevator. The electric machine can in thiscase be either with a gear or without a gear.

The electrical angle of the electric machine refers to the angle valuedetermined by the cycle length of the magnetic flux rotating in theelectric machine. In one embodiment of the invention the cycle length ofthe magnetic flux corresponds here to an electrical angle of 360 degreesin the electric machine.

In the invention alternating electricity excitation signal refers to analternating electricity signal, essentially continuous in terms of itsfundamental wave, that is formed in relation to the electrical angle ofthe electric machine and that changes according to the electrical angle.This type of alternating electricity excitation signal is e.g. anessentially sinusoidal voltage signal or current signal determined as afunction of the electrical angle of the motor. The alternatingelectricity excitation signal thus changes only when the value of theelectrical angle changes; if the value of the electrical angle remainsconstant, the value of the alternating electricity excitation signalalso remains unchanged.

In the invention, pulse-like electrical excitation signal refers to asignal that is formed in a pulse-like way essentially with some constantvalue of the electrical angle of the electric machine.

In the invention, an electrical parameter of the electric machine refersto e.g. the current, voltage and output power of the electric machine.

The frequency converter according to the invention comprises a loadbridge and also a control of the load bridge, for supplying electricitybetween the load bridge and an electric machine connected to the loadbridge. The frequency converter also comprises a determination for atleast one electrical parameter of the aforementioned electric machine,and also a determination for the position of the rotor of theaforementioned electric machine. The load bridge is fitted to supply afirst alternating electricity excitation signal to the aforementionedelectric machine, which first alternating electricity excitation signalis formed in relation to the electrical angle of the electric machine.The frequency converter is fitted to determine the first alternatingelectricity response signal corresponding to the aforementioned firstalternating electricity excitation signal, and the position of the rotoris determined on the basis of the first alternating electricity responsesignal.

In one embodiment of the invention the alternating electricityexcitation signal is made to change according to the electrical angle,and the initial angle of the rotor is determined on the basis of thefirst alternating electricity response signal.

The electrical drive according to the invention comprises an electricmachine and also a frequency converter connected to the electricmachine. The electric machine comprises a machinery brake for preventingmovement of the rotor, and the electrical drive comprises a control ofthe machinery brake. The frequency converter comprises a load bridge andalso a control of the load bridge, for supplying electricity between theload bridge and an electric machine connected to the load bridge. Thefrequency converter comprises a determination for at least oneelectrical parameter of the aforementioned electric machine. Thefrequency converter also comprises a determination for the position ofthe rotor of the aforementioned electric machine. The machinery brake ofthe aforementioned electric machine is during the determination of theposition of the rotor controlled to prevent movement of the rotor, andthe load bridge is fitted to supply a first alternating electricityexcitation signal to the aforementioned electric machine, which firstalternating electricity excitation signal is formed in relation to theelectrical angle of the electric machine. The frequency converter isfitted to determine the first alternating electricity response signalcorresponding to the aforementioned first alternating electricityexcitation signal, and the position of the rotor is determined on thebasis of the first alternating electricity response signal.

In the method according to the invention for determining the position ofthe rotor of an electric machine a first alternating electricityexcitation signal is formed in relation to the electrical angle of theelectric machine; the first alternating electricity excitation signal issupplied to the electric machine; a first alternating electricityresponse signal corresponding to the first alternating electricityexcitation signal is determined; and also the position of the rotor isdetermined on the basis of the first alternating electricity responsesignal.

In one embodiment of the invention the determination of an electricalparameter of the electric machine comprises a current sensor. Thecurrent sensor can be e.g. a current transformer, a Hall sensor, amagneto-resistive sensor or a measuring resistor.

In one embodiment of the invention the determination of an electricalparameter of the electric machine comprises a voltage sensor. Thevoltage sensor can be e.g. a measuring transformer, a linearopto-isolator or a measuring resistor.

In one embodiment of the invention an alternating voltage signal isfitted to be the alternating electricity excitation signal and analternating current signal is fitted to be the alternating electricityresponse signal.

In a second embodiment of the invention an alternating current signal isfitted to be the alternating electricity excitation signal and analternating voltage signal is fitted to be the alternating electricityresponse signal.

When the position of the rotor of the electric machine is determined onthe basis of the first alternating electricity response signal, thenoise of the electric machine caused by the determination is quieterthan prior art, because as presented in the invention the firstalternating electricity excitation signal, which is essentially constantin terms of its fundamental wave, does not produce the same type ofdisturbing noise in the electric machine as e.g. those prior-art methodsin which pulse-like current signals or voltage signals are supplied tothe electric machine as excitation signals. Since, as presented in theinvention, the first alternating electricity excitation signal is formedin relation to the electrical angle of the electric machine, it ispossible to measure the inductance of the magnetic circuit at all theelectrical angle intervals of the electric machine with the firstalternating electricity excitation signal, nor does the measurement needto be repeated with many separate determined values of the electricalangle of the electric machine, which speeds up the measuring.

When a first and a second alternating electricity excitation signal aresupplied to the electric machine, which first and second alternatingelectricity excitation signal are fitted to be of opposite directions intheir rotation directions, the phase shift between the alternatingelectricity excitation signal and the corresponding alternatingelectricity response signal that causes a measuring error can becompensated, because the sign digit of the aforementioned phase shiftchanges as the rotation direction of the alternating electricityexcitation signal changes. In this case the phase shift between thefirst alternating electricity excitation signal and the firstalternating electricity response signal is of the opposite directionwith respect to the phase shift between the second alternatingelectricity excitation signal and the second alternating electricityresponse signal, and the aforementioned phase shifts of oppositedirections can be compensated between each other.

When the initial angle of the rotor of the electric machine isdetermined according to the invention, an absolute sensor can be usedfor the regulation of the electric machine instead of an incrementalsensor. The incremental sensor does not in this case necessarily need tobe fitted directly to the shaft of the electric machine, but instead itcan be fitted e.g. via frictive traction to a rotating part of theelectric machine, such as for instance in connection with the tractionsheave of the hoisting machine of the elevator, which simplifies thefitting of the sensor. In this case e.g. an encoder can also be used asa sensor instead of an absolute sensor, which is generally a morecost-effective solution than an absolute sensor.

In the following, the invention will be described in more detail by theaid of a few examples of its embodiments with reference to the attacheddrawings, wherein

FIG. 1 presents a frequency converter with a voltage intermediatecircuit according to the invention

FIG. 2 presents a second frequency converter according to the invention

FIG. 3 presents a determination of the position of the rotor of anelectric machine according to the invention

FIG. 4 presents the electrical parameters of the electric machine duringone determination of the position of the rotor according to theinvention

FIG. 5 presents the amplitude of the alternating current response signalaccording to the invention as a function of the electrical angle of theelectric machine

FIG. 6 presents the magnetic circuit of an electric machine according tothe invention

FIG. 7 presents a determination of the position of the rotor accordingto prior art

FIG. 1 presents a frequency converter 1 with a voltage intermediatecircuit according to the invention. The frequency converter is fitted tosupply power between the electricity network and the electric motor 4.In this embodiment of the invention the electric motor 4 is apermanently magnetized synchronous motor. The frequency convertercomprises a load bridge 2, which is connected to the electric motor 4for supplying power between the electric motor and the load bridge. Theload bridge 2 comprises controllable solid-state switches. The supplyvoltage of the electric motor 4 is formed by controlling the solid-stateswitches of the load bridge 2 with the control 3 of the load bridge withpulse-width modulation. The frequency converter comprises currentsensors 5, which are fitted in connection with the supply cables of thestator winding for measuring the stator current. A determination 6 forthe position of the rotor of the electric motor is also fitted inconnection with the control 3 of the load bridge.

The load bridge 2 is fitted to supply a first alternating voltageexcitation signal 7 to the electric motor 4. The alternating voltageexcitation signal is formed in relation to the electrical angle 18 ofthe electric machine. The amplitude of the alternating voltageexcitation signal is essentially constant, and the excitation signalchanges as a function of the aforementioned electrical angle 18. Thecurrent of the stator winding of the electric motor produced by thesupplied alternating voltage excitation signal 7 is measured with thecurrent sensors 5. The measured current forms a first alternatingcurrent response signal 9,16 corresponding to the supplied firstalternating voltage excitation signal 7, and the position of the rotorof the electric motor is determined on the basis of the determinedaforementioned first alternating current response signal 9,16.

FIG. 2 presents a second frequency converter 1 according to theinvention. In this embodiment of the invention the load bridge 2 of thefrequency converter is implemented as a matrix converter. The supplyvoltage of the electric motor 4 is in this case formed by controllingthe solid-state switches of the load bridge 2 with the control 3 of theload bridge such that the phase of the electric motor 4 is transientlyconnected to the determined phase of the electricity network forachieving the intended supply voltage of the electric motor 4.

The load bridge 2 is fitted to supply a first alternating voltageexcitation signal 7 to the electric motor 4 according to the embodimentof FIG. 1. The current of the stator winding of the electric motorproduced by the supplied alternating voltage excitation signal 7 is alsomeasured as in the embodiment of FIG. 1. The measured current forms afirst alternating current response signal 9,16 corresponding to thesupplied first alternating voltage excitation signal 7 and the positionof the rotor of the electric motor is determined on the basis of thedetermined aforementioned first alternating current response signal9,16.

The controllable solid-state switches of the load bridge 2 referred toin the invention can be e.g. IGBT transistors, MOSFET transistors orthyristors.

FIG. 3 presents as a block diagram one determination 6 of the positionof the rotor of an electric machine according to the invention. Movementof the rotor of the electric machine 4 is prevented during thedetermination of the position of the rotor. The conversion block 22forms the three-phase supply voltage reference U_(R), U_(S), U_(T) ofthe electric machine from the amplitude reference Û as well as from theelectrical angle reference θ of the electric machine, in which case thethree-phase supply voltage reference is formed as a function of theelectrical angle reference θ. The supply voltage reference U_(R) of theR-phase is in this case of the form: Ûsin θ. The control 3 of the loadbridge controls the solid-state switches of the load bridge 2 accordingto the aforementioned three-phase supply voltage reference U_(R), U_(S),U_(T) for forming the first three-phase alternating voltage excitationsignal 7 for the electric machine. In this embodiment of the inventionthe value of the electrical angle reference θ is changed evenly, inwhich case the rotation speed of the supply voltage reference and at thesame time of the alternating voltage excitation signal 7,8 is constant.The first three-phase alternating current response signal I_(R), I_(S),I_(T) produced in the winding of the electric machine by the firstthree-phase alternating voltage excitation signal is measured 5 as afunction of the electrical angle reference θ of the electric machine.The amplitude of the measured first three-phase alternating currentresponse signal 9,16 is determined 23 with some prior-art method, e.g.by forming a rotation indicator of the current vector for thethree-phase alternating current response signal. The variation of theinductance of the magnetic circuit of the electric machine causes theamplitude Î of also the measured first alternating current responsesignal 9,16 to vary as a function Î(θ) of the electrical angle referenceθ. The impedance of the magnetic circuit also causes a phase differenceto form between the supplied first alternating voltage excitation signal7 and the measured first alternating current response signal 9,16. Tocompensate for the phase difference, the measurement described above isrepeated by supplying a second alternating voltage excitation signal 8as a function of the electrical angle reference θ. The rotationdirection of the second alternating voltage excitation signal 8 isselected to be the opposite to the rotation direction of the firstalternating voltage excitation signal 7, in which case the phasedifference between the first alternating voltage excitation signal 7 andthe first alternating current response signal 9,16 forms to be in theopposite direction compared to the phase difference between the secondalternating voltage excitation signal 8 and the second alternatingcurrent response signal 10, 17. FIG. 4 presents the first alternatingvoltage excitation signal 7 of the R-phase and also the secondalternating voltage excitation signal 8 of the R-phase, which are formedconsecutively. The amplitude of the alternating voltage excitationsignals is otherwise constant, but the second alternating voltageexcitation signal 8 is reduced at the start. This is because the changein the rotation direction of the alternating voltage excitation signalcauses a change phenomenon that affects the current of the winding ofthe electric machine, which is endeavored to be compensated for bytransiently decreasing the amplitude of the voltage of the alternatingvoltage excitation signal 8. FIG. 4 also presents the amplitude of thefirst alternating current response signal 9 corresponding to the firstalternating voltage excitation signal 7 as a function Î(θ) of theelectrical angle, and likewise the amplitude of the second alternatingcurrent response signal 10 corresponding to the second alternatingvoltage excitation signal 8 as a function of the electrical angle. FIG.5 presents in more detail the amplitudes of the first 16 and the second17 alternating current response signals for the cycle length of 0 . . .360 degrees of electrical angle of the electrical angle reference θ ofthe electric machine. The variation in the amplitudes as a function ofthe electrical angle reference θ 18 results from the inductance of themagnetic circuit of the electric machine varying owing to, among otherthings, local saturation of the magnetic circuit. Here, local saturationrefers to the type of saturation phenomenon of a magnetic circuit, whichvaries in relation to the electrical angle of the electric machine. Thiskind of local saturation is caused by, among other things, the permanentmagnets of the rotor, in which case the position of the permanentmagnets of the rotor can be determined utilizing the local saturation.On the other hand, a variation of the geometry of the magnetic circuit,such as e.g. a variation in the length of the air gap of the electricmachine, can also cause a local variation of the inductance of themagnetic circuit of the electric machine. This type of variation in thelength of the air gap occurs e.g. in salient pole electric machines. Thelocal variation of the inductance of the magnetic circuit of theelectric machine caused by a variation of the geometry of the magneticcircuit of an electric machine of the aforementioned type can also beused for the determination of the position of the rotor according to theinvention. In this case the initial angle of the rotor, i.e. theposition of the magnetic poles of the rotor, can thus be determined in asituation where the rotor is locked into its position.

From FIG. 5 it is also possible to detect the phase difference 32between the graphs Î(θ) of the amplitudes of the first alternatingcurrent response signal 16 and the second alternating current responsesignal 17, which phase difference results from the rotation directionsof the first 7 and the second 8 alternating voltage excitation signalbeing in the opposite directions to each other. Since in this case thephase shift between the first alternating voltage excitation signal 7and the first alternating current response signal 16 is in the oppositedirection than the phase difference between the second alternatingvoltage excitation signal 8 and the second alternating current responsesignal 17, the phase difference between the first 16 and the second 17alternating current response signal can be compensated.

The position of the rotor of the electric machine is determined from thefirst and the second alternating current response signal as follows: thefirst and the second alternating current response signal are measured,and on the basis of the measured signals the amplitudes of thealternating current response signals are determined as a function Î(θ)of the electrical angle reference. The determined amplitudes of thealternating current response signals are recorded, in which case thegraphs 16,17 of the amplitudes of the alternating current responsesignals are formed as a function of the electrical angle referenceaccording to FIG. 5. The value of the electrical angle corresponding tothe greatest value of the amplitude is determined from the graphs of theamplitude of the first 16 and the second 17 alternating current responsesignal. This occurs such that the value of the amplitude of the greatestmeasured alternating current response signal is identified, and a curvefit 27, e.g. a parabolic fit, for instance with the least squaresmethod, is formed by means of the measuring points of the environment ofthe greatest value. After this the value 25 of the electrical anglecorresponding to the maximum value of the parabolic fit 27 is resolved.The value 25,26 of the electrical angle is resolved separately for thefirst 16 and the second 17 graph of the amplitude of the alternatingcurrent response signal, and the value 28 of the electrical anglecomprising the position information of the rotor is determined as anaverage value of the value 25 of the electrical angle corresponding tothe maximum value of the graph 16 of the amplitude of the firstalternating current response signal and the value 26 of the electricalangle corresponding to the maximum value of the graph 17 of theamplitude of the second alternating current response signal, in whichcase the phase differences between the alternating voltage excitationsignals 7,8 and the alternating current response signals arecompensated. In this case the determined value 28 of the electricalangle comprising the position information of the rotor corresponds tothe point in the rotor, as presented in FIG. 6, in which the directionof the magnetization 30 produced by the stator current is convergentwith respect to the flux of the rotor magnet 29.

FIG. 7 presents a determination of the position of the rotor accordingto prior art. In this case the load bridge 2 is fitted to supply apulse-like electrical excitation signal 19 with the determined values ofthe electrical angle θ of the electric machine, and the frequencyconverter is fitted to determine a plurality of pulse-like electricalresponse signals corresponding to the aforementioned pulse-likeelectrical excitation signals. The frequency converter is further fittedto determine a reference point 28 for the position of the rotor of theelectric machine on the basis of the aforementioned pulse-likeelectrical response signals.

In one embodiment of the invention a load bridge 2 is fitted to supply asecond alternating electricity excitation signal 8 to the aforementionedelectric machine, and the second alternating electricity excitationsignal is formed in relation to the electrical angle θ 18 of theelectric machine. In this case the phase shift 20 of the secondalternating electricity response signal 10, 17 corresponding to thesecond alternating electricity excitation signal 8 is determined on thebasis of the reference point 28 of the position of the rotor of theelectric machine and the second alternating electricity response signal17 The aforementioned phase shift 20 of the second alternatingelectricity response signal 10, 17 corresponding to the secondalternating electricity excitation signal 8 is presented in FIG. 5.

The invention is described above by the aid of a few examples of itsembodiment. It is obvious to the person skilled in the art that theinvention is not limited to the embodiments described above, but thatmany other applications are possible within the scope of the inventiveconcept defined by the claims presented below.

It is obvious to the person skilled in the art that the first and thesecond electrical excitation signal, such as the first and the secondalternating electricity excitation signal, can be combined into the sameexcitation signal, e.g. by combining the electrical first and secondexcitation signal consecutively. In this case the first and the secondelectrical response signal can also be determined as a combinedelectrical response signal in response to the combined excitationsignal.

It is further obvious to the person skilled in the art that the methodaccording to the invention for determining the position of the rotor ofan electric machine can be performed using different measuring apparatussolutions, and that in this case some other electricity supply solutionthan a frequency converter can also be used for supplying the excitationsignal to the electric machine.

1. A frequency converter, comprising a load bridge and also a control ofthe load bridge, for supplying electricity between the load bridge andan electric machine connected to the load bridge; and which frequencyconverter comprises a determination for at least one electricalparameter of the aforementioned electric machine, and which frequencyconverter comprises a determination for the position of the rotor of theelectric machine; wherein a load bridge is fitted to supply to theaforementioned electric machine a first, essentially sinusoidalalternating electricity excitation signal, having an essentiallyconstant amplitude, and being essentially continuous in term of itsfundamental wave, and which first alternating electricity excitationsignal is formed as a function of the electrical angle reference θ ofthe electric machine, by changing the value of the electrical anglereference θ, and in that the frequency converter is fitted to determineas a function of said electrical angle reference θ a first alternatingelectricity response signal corresponding to the aforementioned firstalternating electricity excitation signal, and in that the initial angleof the rotor is determined on the basis of the first alternatingelectricity response signal.
 2. The frequency converter according toclaim 1, wherein the alternating electricity excitation signal is madeto change according to the electrical angle; and in that the initialangle of the rotor is determined on the basis of the first alternatingelectricity response signal.
 3. The frequency converter according toclaim 1, wherein the load bridge is fitted to supply a first and asecond alternating electricity excitation signal to the aforementionedelectric machine, which first and second alternating electricityexcitation signal are formed in relation to the electrical angle of theelectric machine, and which first and second alternating electricityexcitation signal are fitted to be of opposite directions in theirrotation direction, and in that the position of the rotor is determinedon the basis of the first and the second alternating electricityresponse signals corresponding to the aforementioned first and secondalternating electricity excitation signals.
 4. The frequency converteraccording to claim 1, wherein the load bridge is fitted to supply apulse-like electrical excitation signal with the determined values ofthe electrical angle of the electric machine to the aforementionedelectrical machine, and in that the frequency converter is fitted todetermine a plurality of pulse-like electrical response signalscorresponding to the aforementioned pulse-like electrical excitationsignals; and in that the frequency converter is further fitted todetermine a reference point for the position of the rotor of theelectric machine on the basis of the aforementioned pulse-likeelectrical response signals; and in that the load bridge is fitted tosupply a second alternating electricity excitation signal to theaforementioned electric machine; which second alternating electricityexcitation signal is formed in relation to the electrical angle of theelectric machine; and in that the phase shift of the second alternatingelectricity response signal corresponding to the second alternatingelectricity excitation signal is determined on the basis of thereference point of the position of the rotor of the electric machine andalso on the basis of the second alternating electricity response signal.5. The frequency converter according to claim 1, wherein the frequencyconverter comprises an input for the signal expressing the operatingstatus of the electric machine, and in that the position of the rotor ofthe electric machine is determined in an operating status in whichmovement of the rotor is prevented.
 6. An electrical drive whichcomprises an electric machine and also a frequency converter connectedto the electric machine; which electric machine comprises a machinerybrake for preventing movement of the rotor, and which electrical drivecomprises a control of the machinery brake; and which frequencyconverter comprises a load bridge and also a control of the load bridgefor supplying electricity between the load bridge and the electricmachine connected to the load bridge; and which frequency convertercomprises a determination for at least one electrical parameter of theaforementioned electric machine, and which frequency converter comprisesa determination for the position of the rotor of the aforementionedelectric machine; wherein the machinery brake of the electric machine isduring the determination of the position of the rotor controlled toprevent movement of the rotor, and in that the load bridge is fitted tosupply to the aforementioned electric machine a first, essentiallysinusoidal alternating electricity excitation signal, having anessentially constant amplitude, and being essentially continuous in termof its fundamental wave, which first alternating electricity excitationsignal is formed as a function of the electrical angle reference θ ofthe electric machine, by changing the value of the electrical anglereference θ, and in that the frequency converter is fitted to determineas a function of said electrical angle reference θ the first alternatingelectricity response signal corresponding to the first alternatingelectricity excitation signal, and in that the initial angle of therotor is determined on the basis of the first alternating electricityresponse signal.
 7. The electric drive according to claim 6, wherein thealternating electricity excitation signal is made to change according tothe electrical angle; and in that the initial angle of the rotor isdetermined on the basis of the first alternating electricity responsesignal.
 8. A method for determining the position of the rotor of anelectric machine wherein: a first, essentially sinusoidal alternatingelectricity excitation signal, having an essentially constant amplitude,and being essentially continuous in term of its fundamental wave isformed as a function of the electrical angle reference θ of the electricmachine, by changing the value of the electrical angle reference θ, thefirst alternating electricity excitation signal is supplied to theelectric machine, a first alternating electricity response signalcorresponding to the first alternating electricity excitation signal isdetermined as a function of said electrical angle reference θ, and theinitial angle of the rotor is determined on the basis of the firstalternating electricity response signal.
 9. The method according toclaim 8, wherein: the alternating electricity excitation signal is madeto change according to the electrical angle, and the initial angle ofthe rotor is determined on the basis of the first alternatingelectricity response signal.
 10. The method according to claim 8,wherein: a first alternating electricity excitation signal is formed inrelation to the electrical angle of the electric machine, a secondalternating electricity excitation signal is formed in relation to theelectrical angle of the electric machine, the first and the secondalternating electricity excitation signal are fitted to be of oppositedirections in their direction of rotation the first and a secondalternating electricity excitation signal are supplied to the electricmachine, a first alternating electricity response signal correspondingto the first alternating electricity excitation signal is determined, asecond alternating electricity response signal corresponding to thesecond alternating electricity excitation signal is determined, and theposition of the rotor is determined on the basis of the firstalternating electricity response signal and the second alternatingelectricity response signal.
 11. The method according to claim 9,wherein: a pulse-like electrical excitation signal with the values ofthe electrical angle of the electric machine is formed, and theaforementioned pulse-like electrical excitation signal is supplied tothe electric machine, a plurality of electrical response signalscorresponding to the aforementioned pulse-like electrical excitationsignals are determined, a reference point for the position of the rotorof the electric machine is determined on the basis of the aforementionedpulse-like electrical response signals, a second alternating electricityexcitation signal is formed in relation to the electrical angle of theelectric machine, and the aforementioned second alternating electricityexcitation signal is supplied to the electric machine, and the phaseshift of the second alternating electricity response signalcorresponding to the second alternating electricity excitation signal isdetermined on the basis of the reference point of the position of therotor of the electric machine and the second alternating electricityresponse signal.
 12. The method according to claim 8, wherein: movementof the rotor of the electric machine is prevented during thedetermination of the position of the rotor.
 13. The frequency converteraccording to claim 7, wherein the alternating electricity excitationsignal is made to change according to the electrical angle, and theinitial angle of the rotor is determined on the basis of the firstalternating electricity response signal.
 14. The frequency converteraccording to claim 2, wherein the load bridge is fitted to supply afirst and a second alternating electricity excitation signal to theaforementioned electric machine, which first and second alternatingelectricity excitation signal are formed in relation to the electricalangle of the electric machine, and which first and second alternatingelectricity excitation signal are fitted to be of opposite directions intheir rotation direction, and in that the position of the rotor isdetermined on the basis of the first and the second alternatingelectricity response signals corresponding to the aforementioned firstand second alternating electricity excitation signals.
 15. The frequencyconverter according to claim 2, wherein the load bridge is fitted tosupply a pulse-like electrical excitation signal with the determinedvalues of the electrical angle of the electric machine to theaforementioned electrical machine, and in that the frequency converteris fitted to determine a plurality of pulse-like electrical responsesignals corresponding to the aforementioned pulse-like electricalexcitation signals; and in that the frequency converter is furtherfitted to determine a reference point for the position of the rotor ofthe electric machine on the basis of the aforementioned pulse-likeelectrical response signals; and in that the load bridge is fitted tosupply a second alternating electricity excitation signal to theaforementioned electric machine; which second alternating electricityexcitation signal is formed in relation to the electrical angle of theelectric machine; and in that the phase shift of the second alternatingelectricity response signal corresponding to the second alternatingelectricity excitation signal is determined on the basis of thereference point of the position of the rotor of the electric machine andalso on the basis of the second alternating electricity response signal.16. The frequency converter according to claim 2, wherein the frequencyconverter comprises an input for the signal expressing the operatingstatus of the electric machine, and in that the position of the rotor ofthe electric machine is determined in an operating status in whichmovement of the rotor is prevented.
 17. The frequency converteraccording to claim 3, wherein the frequency converter comprises an inputfor the signal expressing the operating status of the electric machine,and in that the position of the rotor of the electric machine isdetermined in an operating status in which movement of the rotor isprevented.
 18. The frequency converter according to claim 4, wherein thefrequency converter comprises an input for the signal expressing theoperating status of the electric machine, and in that the position ofthe rotor of the electric machine is determined in an operating statusin which movement of the rotor is prevented.
 19. The method according toclaim 9, wherein: a first alternating electricity excitation signal isformed in relation to the electrical angle of the electric machine, asecond alternating electricity excitation signal is formed in relationto the electrical angle of the electric machine, the first and thesecond alternating electricity excitation signal are fitted to be ofopposite directions in their direction of rotation the first and asecond alternating electricity excitation signal are supplied to theelectric machine, a first alternating electricity response signalcorresponding to the first alternating electricity excitation signal isdetermined, a second alternating electricity response signalcorresponding to the second alternating electricity excitation signal isdetermined, and the position of the rotor is determined on the basis ofthe first alternating electricity response signal and the secondalternating electricity response signal.
 20. The method according toclaim 9, wherein: a pulse-like electrical excitation signal with thevalues of the electrical angle of the electric machine is formed, andthe aforementioned pulse-like electrical excitation signal is suppliedto the electric machine, a plurality of electrical response signalscorresponding to the aforementioned pulse-like electrical excitationsignals are determined, a reference point for the position of the rotorof the electric machine is determined on the basis of the aforementionedpulse-like electrical response signals, a second alternating electricityexcitation signal is formed in relation to the electrical angle of theelectric machine, and the aforementioned second alternating electricityexcitation signal is supplied to the electric machine, and the phaseshift of the second alternating electricity response signalcorresponding to the second alternating electricity excitation signal isdetermined on the basis of the reference point of the position of therotor of the electric machine and the second alternating electricityresponse signal.